Principal tanning with mineral tanning substances alone is not sufficient to provide finished leathers with the many properties required of them. In order to enhance specific properties or even to initially obtain them, retanning is added to the principal tanning process. However, in addition to the quality desired by subsequent processors, leather manufacturers must take into account a considerable number of environmental conditions. A principal condition concerns chromium in waste waters introduced by a principal mineral tanning process with various chromium tanning substances. Numerous operating plants are therefore forced either to have the stage of principal tanning performed by installations capable of recovering chromium from residual tanning liquors or to further process wet-blue commercial goods available on the market and already chromium tanned. This, however, does not solve problems of this type. In the course of the further processing of wet-blue goods, an astonishingly large amount of chromium, i.e., too much chromium is still passed from the leather into the waste waters.
The concentration of chromium in the rinsing or retanning liquors is largely dependent on the chromium fixing ability of the retanning agent, which is around 0.1-2 g Cr.sub.2 O.sub.3 /l of the residual liquor in the present state of the art. Because a permissible amount of chromium in waste waters is attained below about 2 mg Cr/l (=2.9 mg Cr.sub.2 O.sub.3 /l), considerable problems arise with the amounts of chromium usually obtained, which exceed the above limit. There has therefore been no lack of attempts to reduce the problem, generally by special processes and/or auxiliary means, but it has not heretofore been possible in actual practice to reduce the chromium concentration of the residual liquors appreciably under 0.1 g Cr.sub.2 O.sub.3 /l without affecting the quality of the leather.
German Offenlegungsschrift No. 27 55 087 describes pretanning of smoothed skins wherein final tanning process concentrations of 0.13 g Cr.sub.2 O.sub.3 /l, 0.28 g Cr.sub.2 O.sub.3 /l and 0.3 g Cr.sub.2 O.sub.3 /l are obtained in the residual liquor by means of polymers of monomers containing carboxyl groups. The polymers are alleged to be suitable for retanning also. In German Offenlegungsschrift No. 31 41 496 copolymers of polyacrylamide and methacrylamide with cationic groups are recommended for retanning, principally for dyeing effects, yet without abnormal chromium fixation effects.
The producers of polymer tanning substances frequently recommend rinsing the leather prior to retanning, which obviously leads to the contamination of waste waters with inadequately fixed chromium. Other types of polymer tanning substances are sensitive to acids and form precipitates so that prior to their use chrome leathers must be additionally neutralized. Depending on the neutralizing agent used, particularly high chromium concentrations may be produced in the waste waters.
In achieving the properties of the leather, external appearance plays an important role. Thus, for example, leathers with the lightest possible original coloring are sought so as to allow very pastel or clear color tones to be dyed in. The same requirements also apply to leathers which are dried without dyeing and stored, to be colored later--depending on the prevailing fashions--by means of the so-called cop dyeing process. The versatility of such leathers with respect to coloring depends very greatly on the presence of a light, neutral base coloring.
The action of the polymer tanning agents may be described in relation to the properties of the leather generally in that the leathers treated with them are soft and more or less full and firmly grained. It should be noted, however, that of polymer tanning agents designated as firm-graining, the term "firm-grained" is relatively flexible and does not approach the usual standard of specially developed auxiliary agents, such as for example that of leather treated with polyphosphates.
However, polyphosphates are at a disadvantage in coloring with very clear dyes, as the leathers receive a greenish color cast as the base color, thereby introducing achromatic components in the complementary range. Furthermore, polyphosphates are not able to produce very strong consumption values required.